In LilyPond itself, it makes sense to document the latest version. If
people read 2.16 documentation, they can't expect to see stuff that is
guaranteed to work under 2.14.
It is not uncommon for some new features to be only discernible from
regtests. That is not really good. This is the current state for
Scheme engravers. It would be good to have some nice examples for
Scheme engravers in the documentation.
This particular case is, in my opinion, too complex for either
documentation or a targeted regtest. It is LSR material, or should
become part of LilyPond proper if one can think of a good way. Note
that we have snippets in the LilyPond documentation/repository as well:
those can use the newest features. That would be the proper place, I
think.
We still need to get Scheme engravers into the main documentation.
http://codereview.appspot.com/5882053/